1. Field of the Invention
The present invention relates to a polarization splitting sheet that passes light of one polarization and reflects light of another polarization, an optical sheet laminate of the aforementioned polarization splitting sheet and a prism sheet, a transmission-type liquid-crystal display apparatus, a planar light source apparatus that is used in illuminating a light transmission-type display element such as used in an advertising panel and which makes use of the aforementioned polarization splitting sheet or optical sheet laminate, and a transmission type display apparatus, such as a liquid-crystal display, which uses this planar light source apparatus.
2. Description of the Related Art
A liquid-crystal display apparatus that is used as a display in, for example, computers and television receivers modulates passes light through a polarizer and uses a liquid-crystal layer to modulate the polarized light obtained in doing so. For example, a typical liquid-crystal display 1 in the prior art is shown in FIG. 12, this being configured so that light that is emitted from a light source 3 of a backlighting apparatus 2 is incident to one end-surface 4A of an approximately planar light guide 4, this light exiting from the light exiting surface 4B, which is the upper surface as shown in this drawing, this light then being diffused by a diffusion sheet 5, after which it is collected by a prism sheet 6, so that it illuminates a liquid-crystal display panel 7 from the rear surface thereof.
In the above-noted backlighting apparatus 2, the light of the light source that is incident at the above-noted one end-surface of 4A is subjected to repeated total reflections within the light guide 4, and part of this light is reflected by the light exiting surface 4B and the light-diffusing element 4D that is disposed at the rear surface 4C, this light passing from the light exiting surface 4B through the diffusion sheet 5 and being shone in the direction of the liquid-crystal display panel 7. Light that is output from the above-noted rear surface 4C of the light guide 4, this being light directed downward in the drawing, is reflected by a reflecting sheet 8 that is disposed therebelow, so that this light is returned once again to the light guide 4.
The above-noted prism sheet is provided with a plurality of unit prisms 6A, these being triangular prisms (having a cross-sectional shape of a triangle or a triangle with a rounded top vertex) or unit lenses having a cross-section shape that is semicircular or semi-elliptical (not shown in the drawing), arranged so that the ridge lines thereof are mutually parallel.
The above-noted liquid-crystal display panel 7 is formed by a liquid-crystal cell 7A and polarizers 7B and 7C, the liquid-crystal cell 7A being configured as a liquid-crystal layer (such as a TN liquid crystal, STN liquid crystal, or a liquid crystal for an IPS or VA), this layer being held between two glass substrates or plastic substrates (neither shown in the drawing), and the above-noted polarizers 7B and 7C hold these substrates between them from the outsides (top and bottom in FIG. 12).
The above-noted liquid-crystal display panel 7, by means of an electric field that is applied to the liquid-crystal layer in the liquid-crystal cell 7A, modulates the condition of the light that passes therethrough, so that, by controlling the relationship of the light-transmitting axes of the polarizers 7B and 7C and the polarized light that passes through the liquid-crystal layer, the amount of light that passes through the liquid-crystal cell 7A is changed, so that information is displayed.
Another prior art liquid-crystal display apparatus 1A, which is shown in FIG. 13, is different from the liquid-crystal display apparatus 1 that is shown in FIG. 12 in that the direction of the prism sheet 6 in the backlighting apparatus 2A is reversed, so that, in contrast to the unit prisms 6A of FIG. 12 which face upward, the unit prisms face downward, and in that a light-scattering light guide 9 is used instead of the light guide 4.
The above-noted light-scattering light guide 9 is made, for example, of a light-transmitting resin which has a substance having a different refractive index at a minute interval therein, so that this itself acts so as to scatter light, thereby making the light-diffusing element 4D that is used in the liquid-crystal display apparatus 1 unnecessary.
Because other elements of the configuration of the above-noted liquid-crystal display apparatus 1A are the same as in the liquid-crystal display apparatus 1, they are assigned the same reference numerals and will not be explicitly described herein.
FIG. 14 shows yet another liquid-crystal display apparatus 1B, in which the backlighting apparatus 2B differs from the backlighting apparatus 2A in that it uses a light guide 9A that has an uneven surface height instead of the light-scattering light guide 9. The light guide 9A with the uneven surface height has the effect of providing minute height variations in the light exiting surface 4B of the transparent light guide 4, so that the light exiting surface 4C itself has the action of diffusing light, thereby changing the direction of travel of light within the light guide 4, and making the light-diffusing element 4D as described above unnecessary. It is also possible to provide the minute height variations in the surface that is on the opposite side of the light exiting surface 4B.
Because other elements of the configuration of the above-noted liquid-crystal display apparatus are the same as in the liquid-crystal display apparatus 1A of FIG. 13, they are assigned the same reference numerals and will not be explicitly described herein.
In all of the liquid-crystal display apparatuses 1, 1A, and 1B, the liquid-crystal cell 7A is held between the polarizers 7B and 7C and, because the polarizers 7B and 7C absorb approximately 50% of the incident light, the efficiency of light usage (transmissivity) is low, thereby making it necessary to shine more light from a light source onto the polarizer 7B, in order to achieve sufficient brightness at the surface of the liquid-crystal display panel 7.
If this is done, however, not only is there an increase in the power consumption of the light source 3 of the backlighting apparatus, but also heat from the light source 3 has an adverse affect on the liquid-crystal layer in the liquid-crystal cell 7A, this leading to such problems as an unclear display on the liquid-crystal display panel 7.
In contrast to the above situation, as disclosed in the Japanese Unexamined Patent Application publications H7-49496 and H8-146416, and in PCT (WO) H9-506985, and as shown in FIG. 15, there is an arrangement in which unpolarized light from a backlighting apparatus 2 (2A, 2B) is splitted into two circularly polarized lights which exhibit rotation directions that are mutually opposite, after which these are either converted to linear polarization, or wherein a polarization splitting sheet 9B is used to split light into two linearly polarized lights which are mutually perpendicular, one of the splitted polarized light components being caused to strike the liquid-crystal display panel 7, and the other polarized light component being returned to the backlighting apparatus 2 (2A, 2B), a reflective sheet (not shown in the drawing) or the like within the backlighting apparatus guiding the light once again to the polarization splitting sheet 9B side for re-use, thereby improving the efficiency of light usage.
In the disclosure in Japanese Unexamined Patent Application publication H7-49496, a polarization splitting sheet that is formed as a laminate of adjacent layers that having mutually different refractive indices is provided at the light exiting surface side of a planar light guide, unpolarized light from the light exiting surface being splitted into two polarized light components that are mutually perpendicular, one of these polarized light components being directed at the liquid-crystal cell, and the other polarized light component being returned to the light source side and caused to be reflected, after which it strikes the polarization splitting sheet once again.
In the disclosure in the Japanese Unexamined Patent Application publication H8-146416, a polarization splitting sheet made of a cholesteric liquid-crystal layer is disposed on the light exiting surface side of a planar light guide, unpolarized light from the light source being splitted into two circularly polarized light components having directions of optical rotation that are mutually opposite, one of these circularly polarized light components being converted to linearly polarized light by means of a quarter wave layer for phase-shifting, after which it is directed so as to strike the liquid-crystal cell, and the other of the circularly polarized light components being returned to the light source side, after which it strikes the polarization splitting sheet once again.
In the disclosure in Japanese Unexamined Patent Application publication PCT (WO) H9-506985, a polarization splitting sheet made of a multilayer drawn film is provided on the light exiting surface side of the backlighting apparatus, the unpolarized light from the light exiting surface being splitted into two polarized light components which are mutually perpendicular, one of these polarized light components being output in the direction of the liquid-crystal cell, and the other polarized light component being returned to the backlighting apparatus and reflected, after which it strikes the polarization splitting sheet once again.
In the disclosure in the Japanese Unexamined Patent Application publication H7-49496, because the light reflected from the polarization splitting sheet is recycled, compared with a liquid-crystal display apparatus in which a light-absorbing polarizer is used, there is a great improvement in the efficiency of light usage (the theoretical maximum value being doubled). However, interference fringes are observed as a repeated pattern of light and dark light between the light-splitting sheet and other optical materials that are adjacent thereto, so that if this light is used to illuminate the liquid-crystal display panel, the image that is formed by the various pixels will be disturbed, thereby causing the problem of a prominent worsening in readability.
This prominent worsening of readability causes a reduction in the quality of the display that is far greater than in a liquid-crystal display apparatus of the past which uses a light-absorbing polarizer and does not make use of a polarization splitting sheet. The reason for this is that, in contrast to the optical reflectivity of a light-absorbing polarizer in the past, which was several percent or lower, the optical reflectivity of a polarization splitting sheet such as described above is approximately 50%.
That is, because a light ray that is reflected from the polarization splitting sheet is recycled to the light source side once again, the amount of light that forms interference between the polarization splitting sheet and other optical materials (such as a prism sheet, a diffusion sheet, a light guide, or a reflective sheet) is approximately 10 times that of the case in which a light-absorbing polarizer is used.
For example, if the reflectivity of a light-absorbing polarizer in the past is 4% and the reflectivity of a polarization splitting sheet is 40%, the amount of light that forms interference between the polarization splitting sheet and other optical materials is 10 times that of the case in which a light-absorbing polarizer is used.
Because the extinction ratio of the polarization splitting sheet 9B is not as great as the extinction ratio of a light-absorbing polarizer of the past, as shown by the double-dot-dash line in FIG. 15, a light-absorbing type of polarizer 9C is sometimes inserted between the polarization splitting sheet 9B and the liquid-crystal display panel 7, in order to improve the extinction ratio. If this is done, however, it was learned that interference fringes occur between the polarization splitting sheet 9B and the polarizer 9C. The reason for this is that, as described earlier, the optical reflectivity of the polarization splitting sheet 9 is very high, this being approximately 50%.
The above-noted interference phenomenon occurs not only with light from the backlighting apparatus side, but can also be caused by external light that strikes the liquid-crystal display panel. That is, external light that strikes the liquid-crystal display panel is reflected by the polarization splitting sheet, interference fringes occurring between this and the light-absorbing polarizer that is disposed nearby.
In contrast to the above-noted situation, for example as shown in the liquid-crystal display apparatus of the Japanese Unexamined Patent Application publication H1-234822, it can be envisioned that a light-scattering surface be formed at the lower surface of the polarizer by forming a light-scattering layer on the above-noted polarization splitting sheet, thereby suppressing the generation of interference fringes. In this case as well, however, there are still the following described three problems.
(1) Loss of Light Intensity
When a light-diffusing layer is provided in a polarization splitting sheet on a backlighting apparatus, on a liquid-crystal display panel, or on both, because light that strikes the polarization splitting sheet is diffused by the light-diffusion layer, the direction of travel of the light beams is scattered into various directions, and the intensity that is observed near the normal direction with respect to the polarization splitting sheet is greatly reduced.
(2) The Occurrence of Flaws
When a light-diffusing layer is provided in a polarization splitting sheet on a backlighting apparatus, on a liquid-crystal display panel, or on both, the raised parts of the surface of the light-diffusion layer which has uneven height cause damage to the prisms, for example, on the prism sheet surface with which they come into contact, these flaws making it impossible to obtain an overall planar light output.
In the case in particular in which the vertex of the prisms of the prism sheet is pointed, with an angle of 100xc2x0 or smaller, force from the raised parts of the uneven surface of the light-diffusion layer is concentrated on the ends (vertices) of the prisms, so that the prisms are particularly susceptible to damage.
(3) Reduction in Degree of Polarization
If a light-diffusion layer is provided on the liquid-crystal display panel side of a polarization splitting sheet, because light that passes through the polarization splitting sheet is diffused by the light-diffusion layer, the direction of the polarization thereof is disturbed, so that the amount of light that was absorbed in a light-absorbing polarizer in the past increases, this representing a commensurate reduction in the efficiency of light usage.
Accordingly, it is an object of the present invention, in consideration of the drawbacks in the prior art as described above, to provide a polarization splitting sheet, an optical sheet laminate, a planar light source, and a transmission-type display apparatus which, without an accompanying loss of intensity, the occurrence of flaws, and a reduction in the degree of polarization, suppresses interference fringes and improves the efficiency of light usage.
The present invention is based on the knowledge that, by selecting the bead diameter of the light-transmitting beads in a coating layer that is used as a light-diffusion layer in a polarization splitting sheet, it is possible to solve the above-noted problems with regard to a reduction in light intensity and a reduction in the degree of polarization, and further that, by selecting the distribution of the light-transmitting beads, it is possible to solve the problem of flaws occurring.
To achieve the above-noted object, a polarization splitting sheet according to the present invention, as recited in claim 1, is formed by a light-transmitting base material that achieves the above-noted object by passing one polarized light component of the incident light and reflecting the other polarized component, one side of this sheet being covered by a coating layer that includes light-transmitting beads having a bead diameter of 1 to 10 xcexcm.
At least part of the light-transmitting beads in the above-noted coating layer can be spherical light-transmitting beads having a half-value width of 1 xcexcm or smaller.
The above-noted polarization splitting sheet can be a laminate of three or more layers mutually adjacent in the thickness direction and having mutually different refractive indices, so that one polarization of the incident P polarization and S polarization light is transmitted, while the other polarization is reflected, so as to separate the two polarizations.
Additionally, the above-noted polarization splitting sheet can be formed so as to include an optical circulation selection layer, made of a cholesteric liquid-crystal layer, so that this cholesteric liquid-crystal layer splits the incident light into one circular polarized light component and a circular polarized light component that is of opposite direction thereto.
Additionally, the above-noted polarization splitting sheet can be formed so as to include a quarter wave layer.
Additionally, the above-noted polarization splitting sheet can have a planar structure of three or more layers, each layer being birefringent, the difference in refractive indices of layers that are mutually adjacent in the thickness direction with respect to one of two light components that have oscillation directions that are mutually perpendicular within a plane being different than the difference in refractive indices of layers mutually adjacent in the thickness direction with respect to the other light component.
To achieve the above-noted object, an optical sheet laminate according to the present invention, as recited in claim 6, has a polarization splitting sheet as described above, and a prism sheet which includes unit prisms or unit lenses laminated with this polarization splitting sheet, the coating layer of the polarization splitting sheet being in physical contact with unit prisms or unit lenses of the adjacently laminated prism sheet.
It is possible to make the vertex angle at the end of the unit prisms or unit lenses that make contact with the polarization splitting sheet 100xc2x0 or smaller.
To achieve the above-noted object, a planar light source apparatus according to the present invention, as recited in claim 8, has a light guide that is in the form of a plate, made of a light-transmitting material, light which is guided into the guide from at least one end surface of this light guide exiting from the light exiting surface thereof, which is the other surface of the light guide, a light source that causes light to strike the above-noted at least one surface of the light guide, and a polarization splitting sheet or an optical sheet laminate such as described above, that is provided on the above-noted light exiting surface of the light guide, and that causes light that is output from the above-noted light exiting surface to strike the above-noted coating layer.
To achieve the above-noted object, another planar light source apparatus according to the present invention, as recited in claim 9, has a light-diffusing sheet, a light source that shines light onto the light-diffusing sheet, a reflector, which is located on the opposite side of the light source from the light-diffusing sheet, and which reflects light from the light source in the direction of the light-diffusing sheet, and a polarization splitting sheet or an optical sheet laminate such as described above, which is disposed so that light that is emitted from the above-noted light-diffusing sheet strikes the above-noted coating layer.
To achieve the above-noted object, a light transmission-type display apparatus according to the present invention, as recited in claim 10, has a planar transmission-type display element, and a planar light source apparatus such as described above, which is provided on the rear surface of this light transmission-type display element, and which illuminates the light transmission-type display element from the rear thereof, with the light that it emits.
In the present invention, by including light-transmitting beads having a bead diameter in the range of 1 to 10 xcexcm in the coating layer that is formed on the surface of the polarization splitting sheet, it is possible to avoid interference with respect to adjacent optical material, and to solve the problems of a reduction in intensity and a reduction in the degree of polarization. By making the half-value width of the bead diameter distribution of these beads 1 xcexcm or smaller, it is possible to prevent damage flaws occurring because of stress concentrations.